1. Field of the Invention
The present invention relates to the field of computer animation and, in particular, to a system and method for coherent noise for non-photorealistic rendering.
2. Description of the Related Art
A wide variety of non-photorealistic rendering techniques make use of random variation in the placement or appearance of primitives. However, many conventional techniques suffer from the “shower-door” effect—the illusion that the random variation exists on a piece of glass through which the scene is being viewed.
Two different conventional techniques have been used to avoid the shower-door effect. Using a first conventional approach, primitives of the non-photorealistic rendering are generated in three-dimensions (3D) and attached to specific geometry. However, if the primitives are placed on 3D objects, then their spacing in the image plane will depend on the distance to the observer, the perspective transformation, and the field of view. If any of these parameters change, so will the rendering. It is difficult to disguise this to an observer. The appearance reflects the computation: it generally looks like the strokes or other rendering primitives exist on the surface of 3D objects instead of representing a unified 2D rendering in the image plane in the style of a traditional artist.
Using a second conventional approach, primitives of the non-photorealistic rendering are generated in 2D and moved according to an optic flow. These approaches also suffer from various limitations. The main problem is that computing optic flow from an image sequence is fundamentally under-constrained and hard to resolve. Examples of problems that occur are textureless regions, motion discontinuities, occlusions, large motions, small objects, non-rigid motion, mixed pixels, changes in illumination, non-Lambertian reflectance, motion blur, and camera noise, among others. Inaccuracies in the optic flow field can result in very unsatisfactory results.
As the foregoing illustrates, there is a need in the art for an improved technique that addresses the limitations of current approaches set forth above.